JP4231466B2 - Soft nitriding steel with excellent wear resistance - Google Patents

Description

  The present invention belongs to nitriding steel used for automobile parts, bicycles, various parts with rotation, or various machine parts that are easily subjected to impact load, and is particularly represented by parts such as automobile transmissions. In addition, both press formability before nitriding treatment and wear resistance after nitriding treatment are excellent on the premise that they are applied to parts to which properties are imparted by further soft nitriding after press forming. Related to nitrocarburized steel.

The nitriding method, which is known as a method of steel surface hardening, generally heats steel parts obtained by forming to a temperature range of Ac ₁ or lower in an atmosphere of nitrogen-containing gas such as NH ₃ to form a surface layer portion. The active layer is diffused to form a stable nitride to harden the surface layer.

  However, at present, the hardening mechanism of the surface layer portion is not well understood. For example, nitrogen atoms form nitrides between Al, Cr, Ti, V, Mo, etc. in the steel, and these dislocations ( There are several hardening mechanism theories such as the theory of having some kind of interference effect with (slip) and the theory of being due to the strain of the nitride itself in the steel.

  As a method for hardening the steel surface layer by such a nitriding method, for example, in Patent Document 1 and Patent Document 2, C: 0.1 to 0.5% of a nitride forming element of Al or Cr in steel of 0.1 to 0.5% is used. Soft-nitriding low alloy steels that have been nitridated at a specified temperature condition have been proposed, both of which have a high C content in the steel and in order to add more elements that form nitrides during nitriding However, the workability of the steel material is inferior, and there is a drawback that the application is limited mainly to tools and structural parts by cutting.

  On the other hand, Patent Document 3 proposes a manufacturing method of extremely low C-IF (Interstitial Free) steel sheet for soft nitriding, which is excellent in deep drawability and nitriding treatment, as a steel for automobiles and machine parts that can be press formed. Has been. However, such conventional steel sheets for soft nitriding have problems such as non-uniform surface properties after nitriding treatment and difficulty in obtaining sufficient strength.

  On the other hand, in Patent Documents 4 to 6 and the like, paying attention to Cu as an additive element, sufficient workability can be ensured by solid-dissolving Cu in steel at the time of steel plate production, and at the time of nitriding treatment, There has been proposed a hot-rolled steel sheet for nitriding treatment in which strength is improved by Cu precipitation in the inside and surface hardening is possible.

  Among these methods, the method disclosed in Patent Document 6 uses automotive parts, ensures the stretch flangeability and hole expandability of the steel sheet required during press forming before nitriding, and nitrides the steel sheet. There has been proposed a steel plate having excellent surface wear resistance in which the hardness is reduced in the thickness direction from the surface after the treatment.

  However, in order to further improve the wear resistance, it is ideal to cure from the outermost surface to the inside. In order to form a hardened layer with sufficient thickness, it is necessary to sufficiently diffuse nitrogen from the steel surface to the plate thickness direction. As nitrocarburizing treatment conditions, the treatment time must be increased or the heating temperature must be raised. Don't be.

  However, an increase in processing time and heating temperature during soft nitriding are not preferable from the viewpoint of improving productivity and reducing manufacturing costs in the manufacture of soft nitriding steel.

  From the current state of the prior art, the nitriding treatment can ensure the diffusion amount of nitrogen into the steel even under the soft nitriding treatment conditions at a lower temperature or in a shorter time and has a sufficient hardened layer in the depth direction from the surface. A nitrocarburized steel having excellent press formability and excellent surface wear resistance after nitriding has been desired.

JP 59-31850 A JP 59-50158 Japanese Patent Laid-Open No. 9-25517 Japanese Patent Laid-Open No. 3-122254 JP-A-11-199970 JP 2003-119548 A

  In the above-mentioned conventional technology, it is difficult to achieve the target of high strength required for recent automobile parts, for example, the hardness of 0.25 mm in the depth direction from the outermost surface is 515 Hv or more, or the target is satisfied. In order to manufacture the nitrocarburized steel, there has been a problem of reducing productivity and cost.

  In view of the current state of the prior art, the present invention is mainly required as an automobile part without reducing productivity and cost, such as stretch flangeability and hole expandability before soft nitriding. An object of the present invention is to provide a nitrocarburized steel excellent in press formability and excellent in wear resistance in which a hardened layer is formed with a sufficient thickness from the surface after the nitrocarburizing treatment.

  The steel of the present invention satisfies the target hardness of 515 Hv or more in terms of Vickers hardness at a depth of 0.25 mm from the outermost surface of the steel in order to sufficiently improve the wear resistance required for parts for automobiles or machine structures. Nitrocarburized steel having a hardened layer, and for that purpose, in particular, the content of nitriding components in the steel is specified, and nitriding at a depth of 0.25 mm from the outermost surface of the steel formed by soft nitriding It is a nitrocarburized steel that defines the precipitation form of the product.

  As for the above steel outermost surface, an iron nitride layer of about several tens of μm is formed on the outermost surface layer portion after the soft nitriding treatment depending on the surface treatment conditions. Therefore, the steel surface including this is defined as the outermost surface. To do.

  That is, the gist of the present invention is as follows.

(1) In mass%,
C: 0.001 to 0.005%,
Si: 0.03 to 0.5%,
Mn: 0.1 to 1.0%,
Al: 0.015% to 0.1%,
Ti: 0.03-0.1%,
Cr: 0.4 to 1.4% is contained,
P: Cr limited to 0.035% or less, the balance being Fe and unavoidable impurities, and further depositing plate-like on the {001} face in the ferrite crystal at a depth of 0.25 mm from the steel outermost surface A nitrocarburized steel excellent in wear resistance, characterized by having a nitride plate size of 5 to 10 nm and a Cr nitride number density of 1 × 10 ¹⁷ cm ⁻³ or more.

  (2) The ratio of N to Cr in the Cr nitride (N / Cr) is within the range of 0.5 to 0.8. Excellent nitrocarburized steel.

(3) The nitrocarburized steel excellent in wear resistance according to (1) or (2) above, wherein the number density of the Cr nitride is 2 × 10 ¹⁷ cm ⁻³ or more.

(4) Furthermore, in mass%,
Cu: 0.8 to 2.0%,
The nitrocarburized steel excellent in wear resistance according to any one of (1) to (3) above, containing Ni: 0.5 × [Cu] to 1.5%.
However, said [Cu] shows Cu content (mass%).

(5) Furthermore, in mass%,
The nitrocarburized steel excellent in wear resistance according to any one of the above (1) to (4), wherein O is limited to 0.004% or less.

  By applying the present invention, it is excellent in press formability such as stretch flangeability and hole expansibility before soft nitriding without lowering productivity and cost as compared with the prior art, and surface after soft nitriding Therefore, it is possible to provide a nitrocarburized steel excellent in wear resistance in which a hardened layer is formed with a sufficient thickness.

  In the present invention, there is a technical limitation to the severe parts needs in recent years only by utilizing the conventional fine dispersion technique of nitrides such as Al, Cr, Ti, V, etc. The inventors have intensively studied various conditions under which a steel plate excellent in press workability is hardened in a sufficient region from the surface layer by soft nitriding treatment and the wear resistance required for parts is improved.

  In particular, focusing on the types and composition of nitrides produced in steel by nitriding, the number density, distribution, and consistency with the parent phase, and optimizing them within a certain range, it is possible to meet strict needs. It has been determined that a surface strengthening technology that can be handled is possible.

  Furthermore, as a result of investigating the behavior of carbonitrides and other precipitates for the purpose of controlling the strength of the steel parts themselves, it was found that the strength design inside the steel materials can be performed independently of the control of the surface hardness.

  Hereinafter, the present invention will be described in detail.

  First, the component composition of the steel of the present invention is limited for the following reasons. “%” Shown below means “% by mass” unless otherwise specified.

  C is made 0.001 to 0.005%. When C in the steel is excessively contained, carbides are formed, and the ductility and press formability of the steel are lowered. Therefore, the upper limit of the content is set to 0.005%. The lower the C content, the better the ductility and press formability. However, if the content is lower than 0.001%, the manufacturing cost increases due to decarburization treatment, so the lower limit is 0.001%. did.

  Since Si has a function of improving the strength of the base material by a solid solution strengthening mechanism as a deoxidizer, the lower limit of its content is set to 0.03% in order to use the effect. On the other hand, in order to prevent deterioration of workability due to excessive addition of Si, the upper limit of the content was limited to 0.5%.

  Mn reacts with S in the steel to form MnS, thereby suppressing cracking and central segregation at high temperatures when manufacturing the steel, so the lower limit of its content is 0.1% To do. In addition, since Mn has a high solid solution strengthening capability, its content can be improved as necessary. However, if the content exceeds 1.0%, the ductility is greatly reduced. Is set to 1.0%.

  P is generally known as one of the solid solution strengthening elements and is allowed to be contained up to a certain extent. However, in the present invention, the upper limit of the content is set to 0.035% in order to suppress the deterioration of ductility. Restrict.

  Al is an element that is effective as a deoxidizer in the same manner as Si. In addition, Al nitride is precipitated during the soft nitriding treatment and contributes to the surface hardening of the steel. In order to fully exhibit these effects, the lower limit of the content is made 0.015%. However, when the Al content exceeds 0.1%, only the steel surface layer portion is cured, and nitrogen diffusion into the steel is suppressed, and a sufficiently thick cured layer cannot be obtained. The upper limit was set to 0.1%.

  Ti forms unnecessary C and N precipitates in steel and contributes to improving ductility by reducing solute C and solute N in steel, so the lower limit of its content is 0.03%. And However, excessive addition of Ti causes an increase in the amount of solid solution Ti and causes a decrease in ductility, and like Al, only the steel surface layer part hardens and suppresses nitrogen diffusion to the inside. The upper limit was set to 0.1%.

  Cr is an element that plays a central role in the present invention, and at the time of soft nitriding, fine nitriding precipitates are formed at a sufficient depth from the steel resurface, and dislocations move in the steel crystal structure. By suppressing, it has the effect | action which raises the hardness of a surface layer part.

  In order to fully exhibit this effect | action and effect, the minimum of the content shall be 0.4%. More preferably, the content is 0.7%. However, if the Cr content is too high, the hardness of only the extreme surface layer portion of the steel will increase, and nitrogen diffusion into the inside will be suppressed, and internal hardening will be delayed, making it impossible to obtain a hardened layer with sufficient thickness. The upper limit of the content is 1.4%. More preferably, the content is 1.2%.

  The steel of the present invention contains the above components as basic components, and by specifying the content thereof, the steel according to the present invention is excellent in press formability such as ductility and hole expansibility before nitriding, and surface after nitriding Therefore, fine nitride as described later can be deposited with high density in a sufficient thickness region and cured.

  The steel of the present invention can contain the following components as necessary for the purpose of improving other properties within a range that does not impair the steel material properties that are the subject.

  Cu is known as an element having an action of precipitating Cu particles in steel during nitriding treatment to improve the strength (see Patent Document 6). However, in the fine nitride formation region of the surface layer portion, it has been found that the precipitated Cu particles hardly affect the hardening. In this region, nitride grows preferentially using Cu particles precipitated during the heat treatment as nucleation sites.

  Therefore, since the Cu particles always form a pair with the nitride, the total number density of precipitates contributing to hardening is almost determined by the nitride. This finding was first discovered by three-dimensional atom probe (3D-AP) analysis. Therefore, the precipitated Cu particles do not contribute to hardening in the fine nitride formation region, and increase only the strength inside the steel where fine nitride is not formed.

  Therefore, in the present invention, it is not necessary to add Cu for improving the hardness of the surface layer portion, and in order to obtain a strength improving effect by Cu precipitation inside (center side) from the surface layer where fine nitride formation such as Cr is not formed. If necessary, Cu is added.

  In order to sufficiently obtain the strength improvement effect by Cu precipitation inside the steel (center side), the lower limit of the Cu content is preferably set to 0.8%. On the other hand, since the effect is saturated by excessive addition, the upper limit of the content is set to 2.0%, preferably 1.5% in consideration of cost.

  Ni is an element that is not essential for the formation of a hardened layer on the steel surface layer. However, when Cu is added, in order to avoid embrittlement cracking caused by Cu during hot rolling, depending on the Cu content. It is preferable to add. In order to obtain this effect sufficiently, it is necessary to add 0.5 times or more the Cu content by mass ratio.

  Therefore, the lower limit of the Ni content is preferably 0.5 × [Cu]. In addition, said [Cu] shows Cu content (mass%). On the other hand, if the content exceeds 1.5%, ductility is lowered, so the upper limit is made 1.5%.

  For these reasons, in the present invention, in order to further improve the strength of the steel material, Cu is added in the range of 0.8 to 2.0%, and hot-rolled embrittlement cracks caused by Cu are avoided at that time. Therefore, Ni is added in the range of 0.5 × (Cu) to 1.5%.

  In the present invention, it is also important to reduce the amount of oxide formed in the steel as much as possible. Particularly, in order to improve stretch flangeability and hole expansibility among press formability, O is set to 0. It is preferable to set it to 0.004% or less.

  Further, other elements other than those specified above are treated as inevitable impurities for achieving the object of the present invention, but more preferably, the content is regulated as follows.

  For example, trace elements such as Sn mixed when scrap is partially used as a raw material do not particularly affect the material characteristics of the steel of the present invention, and thus do not affect the effects of the present invention.

  However, elements having a high affinity with N, such as V, Mo, Nb, Zr, etc., easily form nitrides by nitriding, and adversely affect steel properties such as hardening of the extreme surface layer. It is more preferable to prevent the mixing of these elements as much as possible and to make each content 0.01% or less.

  Further, N contained as an inevitable component in the steel before soft nitriding has a relatively high solid solubility in ferritic iron. If the amount of solute N in the steel is large, the ductility deteriorates and the press productivity deteriorates, which is not preferable. Therefore, the N content before soft nitriding is limited to 0.005% or less, more preferably 0.003% or less.

  After soft nitriding, N that has entered and diffused from the surface during nitriding contributes to surface hardening by precipitating fine and high-density nitride such as Cr and fixing dislocations in steel. This is not the case.

  S lowers the workability and adversely affects from the viewpoint of corrosion resistance and the like. Therefore, it is preferable to reduce the content thereof as much as possible. However, since S is mixed at the stage of dissolution, the upper limit of the content is set to 0.00. It is more preferable to set it to 03% or less.

  In the present invention, in addition to defining the component composition in steel within a preferable range as described above, the form of nitride in the steel surface layer portion formed after nitriding of steel is defined for the following reason.

  The present inventors have used a transmission electron microscope (TEM) and a three-dimensional atom probe apparatus (3D-AP) for the steel surface layer portion after nitriding treatment of steels having different component compositions under different nitriding treatment conditions. Analysis and detailed examination of the optimum form of precipitated nitride from the viewpoint of the hardening characteristics of the surface layer.

  As a result, in order to improve the hardening characteristics of the surface layer portion of the steel, among nitrides precipitated in the deeper region from the steel outermost surface by the nitriding treatment, the region having a depth of at least 0.25 mm, particularly in the ferrite crystal That the size and number density of Cr nitride deposited on the {001} surface of Nd and the ratio of N to Cr in the nitride (N / Cr) are important conditions did.

  As described above, the steel of the present invention contains Cr in the steel, and in the nitriding treatment, nitrogen enters and diffuses into the steel from the surface to form nitrides and nitride-forming elements such as Cr in the steel. .

  At this time, Cr has a slightly smaller affinity with nitrogen than V and the like used in conventional nitriding steel, so when an appropriate amount is added, nitride is formed only at the extreme surface layer, and thereafter The factor of inhibiting the internal diffusion of nitrogen is small, and nitride can be effectively generated from the outermost surface of the steel to a sufficient depth region.

  However, in the case where an element having a high affinity with nitrogen such as V is added in combination, these elements and precipitates are formed in preference to Cr, so that only the extreme surface layer portion is likely to be hardened and sufficient. It becomes difficult to form a cured layer having a thickness.

  In the present invention, the target hardness at a depth of 0.25 mm from the steel outermost surface is set to Vickers hardness (100 g load) in order to sufficiently improve the wear resistance required for parts for automobiles or machine structures. Therefore, it is necessary to define the precipitation form of Cr nitride at a depth of 0.25 mm from the outermost surface of the steel as follows.

  The Cr nitride deposited on the {001} plane in the ferrite crystal has a plate shape, so that a large matching strain is generated in the crystal lattice of the ferrite and effectively acts on the increase in hardness.

  In order to effectively exhibit this action, the size of the Cr nitride needs to be 5 to 10 nm in the plate-like major axis direction. If the size in the plate-like major axis direction is less than 5 nm, a sufficiently large matching strain cannot be generated in the ferrite crystal lattice, and conversely, if it is larger than 10 nm, the incompatibility becomes large. , The hardness will decrease. For this reason, the size of the Cr nitride is 5 to 10 nm in the plate-like major axis direction.

The precipitated Cr nitride obtained by nitriding the steel of the present invention contains Cr and N as main components, but in a small amount, solid solution Al, Ti, Mn in the steel further dissolves, and (Cr , Al, Ti, nitrides showing the crystal composition of Mn) _x N _y is deposited. Since the ratio (N / (Cr + Al + Ti + Mn)) to the total amount of Cr, Al, Ti and Mn is about ½, the stoichiometric composition of the Cr nitride having the above precipitation form is Cr ₂ N. It is preferable that the components are close.

  Depending on the composition ratio (N / (Cr + Al + Ti + Mn)) of N and the total amount of Cr, Al, Ti, and Mn, the form of the Cr-based nitride, and further the density and size of the nitride change.

  As a result of elemental analysis using a 3D-AP apparatus described later, the inventors of the present invention have a ratio (N / Cr) of N content and Cr content that is a main component in the Cr-based nitride is less than 0.5. It was confirmed that stable nitrides were not easily generated, and that the hardening action was reduced because the size of the nitrides was reduced.

  On the other hand, when the ratio of N amount to Cr amount (N / Cr) exceeds 0.8, nitrides in a form different from the above plate-like precipitation form increase, the size of nitride increases, and the density decreases. Therefore, it has also been confirmed that the curing action is reduced.

  For these reasons, in the present invention, the ratio of N amount to Cr amount (N / Cr) in the Cr-based nitride is preferably in the range of 0.5 to 0.8. The ratio of N amount to Cr amount (N / Cr) in the nitride is determined by elemental analysis using a 3D-AP apparatus described later.

Further, in order to set the target hardness at a depth of 0.25 mm from the steel outermost surface to 515 Hv or more in terms of Vickers hardness (100 g load), Cr nitride having the above precipitation form is dispersed in the surface layer with high density. Therefore, the number density of the Cr nitride is set to 1 × 10 ¹⁷ cm ⁻³ or more. Further, in order to further improve the wear resistance improvement by surface hardening, the surface layer is preferably 2 × 10 ¹⁷ cm ⁻³ .

  On the other hand, the ratio of N content to Cr content in Cr nitride (N / Cr) is very small because the size of Cr nitride precipitates is very small, and a method for strictly determining this elemental composition ratio is generally established. There is no current situation. Measurement is impossible even with the EDS (Energy Dispersion X-ray Spectroscopy) method known as the elemental analysis method of TEM.

  Therefore, in the present invention, the ratio of N amount to Cr amount (N / Cr) in the nitride was directly measured using a 3D-AP apparatus capable of directly examining the composition of the fine sized precipitate. In the analysis method using the 3D-AP apparatus, the element species can be determined by field ionizing the constituent atoms of the sample, measuring the time of flight until reaching the detector, and determining the mass-to-charge ratio.

For Cr nitride in steel, N of its constituent atoms is detected mainly as the mass to charge ratio (14) of N ⁺ ions, and Cr is detected as the mass to charge ratio of Cr ²⁺ ions. Moreover, since Cr nitride is also detected as a mass-to-charge ratio of CrN ²⁺ complex ions, this was also added to the elemental analysis. The nitride composition ratio (N / Cr) in the steel of the present invention was determined using this method.

However, W.W. Sha, L .; Chang, G. D. W. Smith, Liuheng and E.M. J. et al. Mittemeier: Surface Science 266 (1992) p416-423. As described therein, a part of N (about 30%) becomes NN ⁺ ions and overlaps with Fe ²⁺ ions at a mass to charge ratio of 28, making analysis impossible.

  Therefore, the estimation of the amount of N by 3D-AP may be underestimated, and the actual (N / Cr) ratio in Cr nitride may be slightly larger than the range defined in the present invention.

  It should be noted that the size of the Cr nitride deposited on the {001} face in the ferrite crystal at a depth of 0.25 mm from the outermost surface of the steel of the present invention described above, its number density, and in the nitride The ratio of N amount to Cr amount (N / Cr) can be quantified by analyzing precipitates of the steel surface hardened layer using, for example, TEM, 3D-AP or the like.

Further, the method for producing the nitrocarburized steel of the present invention does not need to be specified in particular. For example, a conventional nitrocarburized steel using a nitrocarburized steel obtained by a conventionally known production method such as Patent Document 1 is used. A treatment method may be used and the conditions may be adjusted to produce Cr nitride in a predetermined precipitation form at a desired predetermined depth. From the viewpoint of quality and manufacturing cost, a gas soft nitriding method using CO + NH _{3 formed} by thermally decomposing urea is preferable as the nitriding method.

  As preferable nitriding conditions for forming the surface hardened layer of the nitrocarburized steel of the present invention, in particular, increasing the heating temperature during the nitriding treatment or increasing the treatment time increases productivity and cost. . Further, it may cause coarsening of the precipitated nitride, obstruct the generation of matching strain in the ferrite crystal lattice, and conversely cause inconsistency, which may cause a decrease in hardness.

  It is preferable from the viewpoint of productivity improvement and cost reduction that the Cr-containing steel defined in the present invention is soft-nitrided at a low heating temperature and in a short processing time.

  Below, an example of the preferable form for manufacturing this invention nitrocarburized steel is demonstrated. In addition, it is not limited to the following manufacturing methods.

  First, an example of a manufacturing form of a steel material before soft nitriding, that is, a steel material for soft nitriding will be described.

Steel in the above component composition range is made into a slab steel slab by continuous casting or the like, and this steel slab is heated as it is or by reheating to a temperature of 1150 to 1250 ° C., followed by rough rolling and finish rolling. At this time, the finish rolling is preferably finished in the austenite region above the Ar ₃ transformation point from the viewpoint of structure control for obtaining characteristics such as ductility.

  The winding after the finish rolling is performed at 700 ° C. or lower. In addition, when Cu is contained in the steel material by 0.8% or more, if it is slowly cooled after winding, Cu precipitates inside the steel material, and the subsequent workability deteriorates due to its precipitation hardening ability. It is desirable to cool to a temperature range of 400 ° C. or lower at a rate of 100 / ° C. or higher.

  In the case of steel containing less than 0.8% Cu, or component steel not containing Cu, this rapid cooling is not necessary.

  The steel according to the present invention is intended to have properties excellent in press formability such as hole expansibility before soft nitriding, and therefore it is desirable to subject the steel to hot nitriding in the state of a hot-rolled steel sheet.

However, if necessary, cold rolling may be performed subsequent to hot rolling, followed by recrystallization heat treatment, and the cold-rolled steel sheet may be soft-nitrided. However, in this case, in the ultra-low carbon IF steel targeted by the present invention, when the annealing temperature during the recrystallization heat treatment exceeds the Ac ₃ transformation point, the press formability such as ductility and r value deteriorates. It is necessary to consider that the upper limit of the temperature is 900 ° C.

  Further, the overaging condition following the recrystallization step is also set to 490 MPa or less in tensile strength, and in order to ensure the workability of the steel material, the temperature is preferably 350 ° C. or less.

  Next, preferable conditions for nitriding the nitriding steel obtained by these manufacturing methods will be described. Usually, after the steel for nitriding treatment is press-molded into an automobile part or a machine structural part, the surface layer is cured by nitriding the part.

In the gas soft nitriding treatment, the gas soft nitriding treatment is preferably performed in a gas atmosphere of CO ₂ + H ₂ + N ₂ + NH ₃ at a heating temperature of 540 to 630 ° C. and a treatment time of 3 to 5 hours.

  In general, from the viewpoint of productivity improvement and cost reduction, it is desirable to perform the gas soft nitriding treatment at a low temperature and in a short time. Since the steel of the present invention contains a predetermined amount of Cr, it produces Cr nitride from the surface of the steel material to a sufficient depth region even at low temperature and in a short time during nitriding, compared to conventional nitriding steel. However, in order to form the above-described precipitation form Cr nitride of the steel of the present invention, for example, it is preferable to perform the following two-stage heat treatment.

  That is, in the gas soft nitriding treatment, as the first nitriding treatment, the primary heating temperature: a relatively high temperature of 570 to 630 ° C. and the treatment temperature: within a short time within 1 hour, and then the second As the nitriding treatment, the treatment is preferably performed at a relatively low secondary heating temperature: 540 to 600 ° C. and at a treatment temperature of 2 to 4 hours.

  By using this method, it is possible to suppress inhibition of nitriding diffusion into the steel material due to excessive growth of nitride on the surface layer, and the above-described precipitation form of Cr nitride is sufficiently dense and sufficient from the surface layer. It is possible to generate over a wide depth range.

  Of course, the nitriding treatment need not be limited to the multi-stage soft nitriding treatment as described above, and by adjusting the conditions when nitriding the steel material having the component composition defined in the present invention, the surface hardened layer defined in the present invention is formed. What is necessary is just to be able to form.

  Examples of the present invention will be described together with comparative examples.

  Various steels having the component compositions shown in Table 1 were melted to produce ingots. The analysis values shown in Table 1 are chemical analysis values, and the unit is “mass%”.

  The ingot was heated at 1250 ° C. for 1 hour, and then hot-rolled at a finishing temperature of 930 ° C. and a finishing thickness of 2 mm. Steels A to C and E to L (without addition of Cu) shown in Table 1 were 580 ° C. It was wound up at a temperature to produce a hot rolled steel sheet. Steels D, M, and N (addition of Cu) shown in Table 1 were hot-rolled under the same conditions, then water-cooled to 380 ° C., allowed to cool, and wound to produce a hot-rolled steel sheet.

  Next, No. 5 test piece described in JIS Z 2201 was collected from each hot-rolled sheet, processed, and subjected to a tensile test according to the test method described in JIS Z 2241. Table 2 shows the yield stress (YP) and tensile strength (TS) measured in the tensile test.

In addition, as an index of press formability, a test of total elongation characteristics and hole expansibility was performed, and each was measured. In the hole expansibility test, the hole diameter (d) and the initial hole diameter (when the crack penetrates the plate pressure) are expanded with a 60 ° conical punch with the burr of the punched hole having a diameter of 10 mm outside. d ₀ ) and the ratio (d / d ₀ ). Table 2 shows the measurement results of total elongation (El) and hole expansion value (d / d ₀ ).

As shown in Table 2, all the steels A to D (within the steel component invention) within the range of the component composition defined in the present invention have a hole expansion value (d / d ₀ ) exceeding 2.5, The total elongation (El) is also more than 35% and has excellent press formability.

On the other hand, the steels E to G, M, and N (outside the steel component invention) deviating from the component composition range defined in the present invention have a total elongation (El) of 34% or less and a hole expansion value (d / d ₀ ) of 2. .2 or less, both were insufficient and the press moldability was low.

Steels H to L (outside the steel component invention) are steels that fall outside the range of the component composition defined in the present invention, but the total elongation (El) is 35% or more, and the hole expansion value (d / d ₀ ) is It was 2.5 or more, and the press formability was sufficient.

  Next, steels A to D (within the steel component invention) within the range of the component composition defined by the present invention, which had good workability, and the component composition range defined by the present invention, but the workability was good The steels H to L (outside the steel component invention) were further subjected to soft nitriding treatment, and then the characteristics of the soft nitriding steel were examined.

The nitriding treatment uses a two-step nitriding treatment method. In the gas atmosphere of CO ₂ + H ₂ + N ₂ + NH ₃ , the first nitriding treatment is followed by the second nitriding treatment. It carried out in. The first nitriding treatment and the second nitriding treatment in Table 3 show the heating temperature and the treatment time, respectively. Test No. in the table. Except for 5, a two-step nitriding treatment was performed in which the total treatment time was within 3 hours from the viewpoint of cost reduction.

  Characteristic evaluation of nitrocarburized steel after nitrocarburizing is performed by mechanical polishing from the surface to the desired depth (0.25 mm, 0.8 mm), and then performing a Vickers hardness test with a load of 100 g to determine the Vickers hardness (Hv). Measured and performed. In addition, a sample having a target depth (0.25 mm, 0.8 mm) is prepared by mechanical polishing or electrolytic polishing, and the shape, size, number density, and component ratio of the nitrided precipitate are determined using TEM and 3D-AP. Examined. These results are shown in Table 3.

  1 and FIG. 2 show precipitates observed using TEM and 3D-AP at a depth of 0.1 mm from the outermost surface of steel D shown in Table 3, respectively.

  FIG. 1 is a TEM photograph. From this, it can be seen that plate-like Cr nitride aligned with a parent phase having an average of 7 nm is densely distributed on the {001} plane. Moreover, FIG. 2 is an element map by 3D-AP, and shows that the Cr nitride observed in FIG. 1 is a nitride mainly containing Cr containing Al and Mn.

  Steel D is a steel containing Cu as a selective component together with Cr. From the analysis results of FIGS. 1 and 2, it can be seen that precipitated Cu particles exist in pairs with Cr nitrides (FIG. 2). Inside, arrows indicate pairs). In the element map shown in FIG. 2, each point corresponds to one atom, and from this, the component ratio in Cr nitride can be accurately obtained.

  In Table 3, test no. Examples 1 to 6 are examples in which the same steel A was used and soft nitriding was performed under different nitriding conditions.

Test No. 3 and 4 are precipitation forms of Cr nitride at a depth of 0.25 mm from the outermost surface (plate-like precipitation on the {001} plane, size in the plate-like direction: 5 to 10 nm, ratio of N amount to Cr amount ( N / Cr): 0.5 to 0.8, number density: 1 × 10 ¹⁷ cm ⁻³ or more) is within the scope of the present invention. 1, 2, 5 and 6 show comparative steels that are outside the scope of the present invention.

  In Table 3, when the nitriding temperature of the first stage and the second stage is low, internal diffusion of nitrogen is not sufficiently performed, and the number density of precipitates cannot be sufficiently obtained. Was not obtained. On the other hand, in the nitriding treatment at a high temperature or for a long time, the internal diffusion of N is sufficient, but the coarseness of the precipitates, the number density reduction, the inconsistent change, etc. occur, and the target hardness cannot be obtained.

  In addition, Test No. Nos. 7 to 14 are No. Although it is the same favorable nitriding conditions as 3 (steel A), it is the Example which performed soft nitriding using the steel grade from which steel B-L differs. Test No. Nos. 7 to 9 are invention steels in which the component composition in the steel is within the scope of the present invention, Test No. Nos. 10 to 14 are examples in which soft nitriding treatment was performed under the same conditions using comparative steels outside the scope of the present invention.

  Further, regarding the shape of Cr nitride shown in Table 3, the plate-like precipitate on the {001} surface has its long diameter, the rod-shaped one has its length, the rectangular parallelepiped has the longest one side, and the indefinite one has The diameter of the sphere was expressed as an average value.

  From Table 3, all of the composition of the steel and the precipitation form of Cr nitride at a depth of 0.25 mm from the outermost surface are within the range specified by the present invention in Test Nos. 3, 4, and 7-9. The inventive steel had a sufficient surface layer hardness of 515 Hv or more at 0.25 mm from the outermost surface.

  Of these invention steels, test No. 9 is an invention steel obtained by nitriding steel D to which Cu is added as a selective component together with Cr. The hardness of 0.25 mm depth of the invention steel of this test No. 9 is almost the same as other invention steels to which Cu is not added, but the hardness of 0.8 mm depth is 2 in comparison with other invention steels. It improved about twice.

  As described above, as is apparent from the steel element map obtained by nitriding steel D (FIG. 2), in the surface hardening region where Cr nitride and Cu particles coexist, Cu particles are: Since it is always paired with Cr precipitates, the contribution of hardness improvement by Cu particles is small.

  However, since it is difficult to form Cr nitride on the inner side, it is considered that the improvement in hardness due to single precipitation of Cu particles appears remarkably.

  On the other hand, as test Nos. 12 to 14, test Nos. 3 to 14 were used as steels J to L, which have a greater affinity with N than Cr and to which a large amount of nitride-forming elements such as Al, Ti, and V are added. It is the Example which implemented the nitriding process on the same optimal conditions as (steel A). As a result, only the extreme surface layer portion is excessively hardened, which inhibits nitrogen diffusion into the inside. As a result, a hardened layer having a sufficient thickness is not formed from the outermost surface, and the hardness at a depth of 0.25 mm is low. It was.

  Moreover, test No. 10 uses steel H in which the addition amount of Cr is smaller than the range of the present invention as a comparative steel, and test No. 11 is a steel in which the addition amount of Cr is larger than the range of the present invention as a comparative steel. This is an example in which nitriding was performed under the same optimum conditions as in Test No. 3 (steel A).

  As a result, in all the comparative steels, the density of Cr nitride at a depth of 0.25 mm from the outermost surface was lower than the range of the present invention, and the hardness at a depth of 0.25 mm was lowered.

  As described above, the application of the present invention is superior in press formability such as stretch flangeability and hole expansibility before soft nitriding without lowering productivity and cost as compared with the prior art. After nitriding treatment, it is possible to provide a soft nitriding steel excellent in wear resistance in which a hardened layer is formed with a sufficient thickness from the surface.

  Therefore, according to the present invention, when manufacturing parts for automobiles or machine structures, etc., high quality and low cost products with good wear resistance while maintaining good press workability can be obtained. The contribution of the present invention to the industry is very great.

It is a figure which shows the structure | tissue photograph of the precipitate in surface layer depth 0.1mm observed using the transmission electron microscope (TEM) after soft-nitriding D steel. It is a figure which shows the elemental map of the precipitate in the surface layer depth of 0.1 mm obtained by analyzing D steel after soft-nitriding process using a three-dimensional atom probe apparatus (3D-AP).

Claims (5)

% By mass
C: 0.001 to 0.005%,
Si: 0.03 to 0.5%,
Mn: 0.1 to 1.0%,
Al: 0.015% to 0.1%,
Ti: 0.03-0.1%,
Cr: 0.4 to 1.4% is contained,
P: Cr limited to 0.035% or less, the balance being Fe and unavoidable impurities, and further depositing plate-like on the {001} face in the ferrite crystal at a depth of 0.25 mm from the steel outermost surface A nitrocarburized steel excellent in wear resistance, characterized by having a nitride plate size of 5 to 10 nm and a Cr nitride number density of 1 × 10 ¹⁷ cm ⁻³ or more.   The soft nitriding excellent in wear resistance according to claim 1, wherein the ratio of N amount to Cr amount (N / Cr) in the Cr nitride is in the range of 0.5 to 0.8. Processing steel. 3. The nitrocarburized steel excellent in wear resistance according to claim 1, wherein the Cr nitride has a number density of 2 × 10 ¹⁷ cm ⁻³ or more. Furthermore, in mass%,
Cu: 0.8 to 2.0%,
Ni: 0.5 × [Cu] to 1.5%
The nitrocarburized steel excellent in wear resistance according to claim 1, comprising:
However, said [Cu] shows Cu content (mass%). Furthermore, in mass%,
The nitrocarburized steel excellent in wear resistance according to any one of claims 1 to 4, wherein O is limited to 0.004% or less.